1. Field of the Invention
The present invention relates to a drive control method for an ultrasonic transducer.
2. Description of the Prior Art
Normally, an ultrasonic transducer is preferably driven at the fundamental resonant frequency which is inherent in its vibration mode in order to provide improved electro-mechanical conversion efficiency. However, since the peak of resonance Q is high in general, even when the driving frequency is only slightly shifted from the resonant frequency the conversion efficiency will be significantly decreased. Consequently a driving oscillator with an automatic following apparatus is widely used for automatically detecting the resonant point of the ultrasonic transducer automatically providing subsequent oscillations.
When the resonant length of the mechanical vibratory system including the ultrasonic transducer as well as horns, tools and the like is about one wavelength or less and when the amplitude multiplication factor is not large, no serious problems occur. However, if the resonant length increases beyond one wavelength or if the multiplication factor becomes large, many sub resonant frequency points appear near the fundamental resonant frequency and therefore the oscillation may be transferred to sub resonant points when oscillation starts or when rapid variation of load occurs. This significantly obstructs the reliability of the ultrasonic wave generating apparatus. Furthermore, in such mechanical vibratory system having many sub resonont points, if the horn or tool is replaced by other part of different fundamental resonant frequency, the required resonant frequency selection and the subsequent oscillations are difficult to attain.
A number of systems have been used in practice as automatic following apparatus of resonant frequency. In many cases, vibratory velocity of the ultrasonic transducer is detected and the frequency of the driving signal is controlled so that its phase relationship to the drive voltage or drive current becomes constant. Such detecting methods of vibratory velocity signal include a method wherein a detecting element such as electro-strictive element is attached to part of a mechanical vibrator and the generated voltage is detected and a method wherein different motion signals are detected in differential form corresponding to vibratory stress arranged in a plurality of electro-strictive elements.
An example of the frequency characteristics of the phase relationship of a detecting signal is shown in FIG. 1(a) within frequency characteristics of the amplitude of drive current flowing through a transducer being shown in FIG. 1(b). In FIG. 1(a), the follow control region of the oscillator has the resonant frequency f.sub.0 at the center, the phase lead region at the low frequency side and a phase lag region at the high frequency side and is limited to the region f.sub.1 -f.sub.2, for example. The variation of the resonant frequency within the limited region is followed and driven. If the resonant frequency varies beyond the limited region, that is, if it is transferred to the resonant frequency f.sub.0 as shown in FIG. 2, an abnormal vibratory state as shown in point B of FIG. 2(a) will occur where oscillation is generated at sub resonant point even if the following region of the oscillation is enlarged.
As above described, if the horn or tool connected to the ultrasonic transducer is replaced by various parts, such as the horn or tool which are of a different resonant frequency, the conventional following method cannot detect the fundamental resonant frequency on account of many sub resonant points existing near the fundamental resonant frequency.